Technique and apparatus for measuring and monitoring the mechanical impedance of body tissues and organ systems

ABSTRACT

A technique and apparatus utilizing tuned mechanical and pneumatic principles for assessing the mechanical properties of body tissues and organ systems. The apparatus vibrates at a tuned frequency and is loaded by the specific tissue or organ system being assessed resulting in an impedance and a phase angle shift. From the above data, the resistive and reactive components of the measured impedance may be determined.

United States Patent Alan R. Kahn Cherry Hill:

Warren L. Childs. Willingboro, both of, NJ.

Dec. 9, 1968 Aug. 10, 1971 Health Technology Corporation inventors Appl.No. Filed Patented Assignee TECHNIQUE AND APPARATUS FOR MEASURING ANDMONITORING THE MECHANICAL IMPEDANCE OF BODY TISSUES AND ORGAN SYSTEMS 9Claims, 10 Drawing Fi 11.8. CI 128/108, 73/67.l, 73/67.2 Int. Cl A6lb5/08, GOln 29/00 Field of Search 73/67. I 67.2, 69; 128/2, 2.08

PLIFIE [56] References Cited UNITED STATES PATENTS 2,837,914 6/1958Caldwell 73/67.] 3,410,264 ll/l968 Frederik.... 128/208 X 3,440,8674/1969 Prall et al 73/67.1

Primary Examiner-Channing L. Pace Attorney-Woodcock, Washburn, Kurtz &Mackiewicz ABSTRACT: A technique and apparatus utilizing tunedmechanical and pneumatic principles for assessing the mechanicalproperties of body tissues and organ systems. The apparatus vibrates ata tuned frequency and is loaded by the specific tissue or organ systembeing assessed resulting in an impedance and a phase angle shift. Fromthe above data, the resistive and reactive components of the measuredimpedance may be detennined.

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mvsmoas BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a new technique and apparatus for assessingmechanical properties of various body tissues and organ systems and moreparticularly to breathing apparatus for and the measurement of lungimpedance.

In order to provide a basis for describing the principles andapplicability of this method, it will be helpful to briefly describe therelevant physiological factors. The mechanical act of breathing beginsas a result of the contraction of muscles of the diaphragm and chestwall which generate forces within the chest cavity which tend to changeits volume. However, the viscosity of the lung tissues, the resistanceof the air passages to the free flow of air, and the elasticity of thelung tissues and other structures of the chest all tend to prevent theforces developed by the muscles from changing the shape of the chestcavity. In fact, a certain amount of negative pressure must be developedwithin the chest before these impeding factors yield and permit theprocess of inhalation to take place.

The ability of the lungs to comply with the mechanical forces generatedby the muscular contraction is termed pulmonary compliance and isexpressed in terms of the pressures developed within the cheststructures and the airflows which take place as a result. Pulmonarycompliance is usually determined by performing two measurementssimultaneously. One of these involves the insertion of a partiallyinflated balloon down the throat into the esophagus. The pressure withinthis balloon is coupled via an interconnecting tube to apressure-measuring device and is a reflection of the pressure within thechest. The other measurement is that of the rate of the flow of air inand out of the respiratory system and is achieved by using a flowmeterattached to a face mask or other suitable breathing appliance. Thepotential of the lungs to comply to the forces of breathing can then bedetermined by interpretation of the results of these two measurements.Abnormally low compliance values result from decreased caliber of theair passages as seen in asthmatic conditions, from overinflation of thechest as seen in emphysema, from distention of the capillaries of thepulmonary circulation and the collection of edema fluid in the lungtissue as seen in certain kinds of heart failure and in episodes ofexcessive administration of intravenous fluid. From the compliance data,it is not usually possible to determine which of these factors isresponsible for the problem.

2. Description of Prior Art Quantitative evaluation of the performanceof the breathing mechanism is important to the prevention, diagnosis,and therapy of a wide variety of disease states. Diseases of the lungsthemselves such as emphysema, asthma, and chronic bronchitis causemechanical changed which can potentially be measured. Since thepulmonary circulation is an intimate part of lung structure, certaindiseases of the cardiovascular system are also reflected in changes inthe mechanical properties of the lungs. In addition, when body tissuesare caused to contain excessive quantities of water, fluid accumulationsin the lung tissue cause mechanical changes which are especiallydangerous and frequently cause death in acutely ill persons. This lattercondition is called pulmonary edema.

Present art of performing measurements on the pulmonary system involvesdifficult and complicated procedures which demand considerablycooperation on the part of the patient and which yield nonspecific datafrom which diagnosis is difficult. Present methods are too complicatedfor applications in mass screening and too difficult to be used forcontinuous monitoring purposes. Since they require patient cooperation,they are not applicable to young children or to gravely ill patients whoare comatose or under the effects of sedative medications or anestheticagents. Probably the most useful data is presently derived frompulmonary compliance measurements which are dangerous to perform onacutely ill patients and on newborn children.

Present apparatus state of the art is represented by the DynamicPulmonary Resistance device manufactured by Lexington InstrumentsCorporation, Waltham, Mass. 02154. This device is described in the July1968, edition of Medical Electronics News, page 3.

SUMMARY OF THE INVENTION In accordance with this invention, a rapidlypulsating source of air is applied to the lungs via the upperrespiratory tract. The source of this pulsating air is sensitive to thedegree of restriction to the free flow of air caused by the pulmonarysystem of the patient who is attached. The apparatus is further able toresolve the nature of the impedance to airflow into its purely resistivecomponents caused by restriction of the air passages or by tissueviscosity and its reactive component resulting from the stretching ofelastic structures in the lung and chest walls. The method also includesthe means for processing these data and presenting them in a formatwhich is meaningful to clinical personnel.

The human respiratory tract appears to behave as a tuned pneumaticsystem. This is a result of several factors which influence themechanical act of breathing. The flow of air into or out of the lungs isinitiated by the development of a pressure gradient between theinnermost portions of the lung tissues and the outside world. As aresult of this pressure gradient, there is a tendency for air to flowinto or out of the respiratory system. This flow of air is impeded byseveral factors. First of all, in order to accommodate airflow, therespiratory system must change its physical volume. This requires thatthe bones, muscles, and other tissues involved in respiration mustchange their relative positions. In doing so, a certain amount offriction is involved and some of the energy which is available as aresult of the pressure gradient must be expended to overcome thesefrictional forces. This appears as a simple resistance to airflow. Asecond source of impedance results from the necessity to stretch certainelastic structures in order to change the shape of the lungs. Thesestructures include elastic fibers in the lung tissues themselves, themuscles of the chest wall, and the linings of the upper respiratorytract. Energy absorbed in this manner is not dissipated, but is absorbedin these elastic structures on a temporary basis. For the sake of thisdiscussion, we can call this type of impedance an elastic reactance. Athird source of impedance to airflow results from the mass of aircontained within the breathing pathways. When a fluid such as air isenclosed within a cylindrical tube, it exhibits inertia similar to anymass and tends to resist rapid changes of the position of the air withinthe tube. For the sake of this discussion, we will call this a massreactance and note the fact that while this type of reactance alsostores energy, it behaves in a manner opposite to the elastic reactancepreviously described. As a consequence, when contained within the samemechanical system, elastic reactance and mass reactance tend to cancelone another. Such a system is said to be tuned" and if a sinusoidallyvarying pressure is applied to such a system, it is possible to selectthe frequency of the variation so that these two reactances exactlycancel one another and know that impedance is experienced. Therespiratory tract of human patients comprises such a system ofreactances and frictional resistance and appears to be resonant at afrequency of about 15 cycles per second when pressure is applied from anoutside source. As a result, when a 15 cycle per second sinusoidalvarying pressure is applied to the a subject via a breathing appliance,a normal human patient will exhibit only the frictional resistancepreviously described and the flow resulting from the varying pressurewill appear to be closely in phase with the driving pressure. However,if mechanical changes are produced within the lungs as a result of adisease process, this delicate balance is disturbed and a greaterportion of the applied energy is stored in the stretching of elasticstructures. As a result, the respiratory system is no longer tuned tothe driving frequency and a phase shift develops between the sinusoidalpressure and the resulting sinusoidal airflow.

Accordingly, it is an object of the present invention to provide a newtechnique and apparatus for assessing mechanical properties of variousbody tissues and organ systems which can be performed simply andrapidly, which does not require cooperation on the part of the patient,which, can be used continuously for monitoring purposes, and whichprovides precise data that can be more directly related to specificdisease processes.

It is another object of the present invention to provide a method forconveniently measuring the mechanical properties of the lungs andassociated breathing apparatus for use in screening patients in order todetect illness, in diagnosing the specific problem, in monitoringacutely ill patients, and in following the progress of therapy.

DESCRIPTION OF THE DRAWINGS FIG. 1 represents a specific embodiment ofthe apparatus of the present invention.

FIG. 2 shows an impedance plot from which resistance and reactance maybe determined.

FIGS. 3 and 4 show records plotted from data taken by the apparatus ofthe present invention.

FIG. 5 represents an alternate embodiment of the apparatus of thepresent invention.

FIGS. 6 and 7 represent alternate embodiments of the air driver unit ofthe present invention.

FIG. 8 shows the oscillator circuit used in the embodiments described inFIGS. 1 and 5.

FIG. 9 shows the signal amplifier and amplitude detector circuitry usedin the embodiment described in FIG. 1.

FIG. 10 shows the phase detector circuit used in the embodimentdescribed in FIG. 1.

DESCRIPTION OF A PARTICULAR EMBODIMENT FIG. 1 illustrates a blockdiagram of the apparatus we have constructed to establish thefeasibility and practicality of this invention. Using FIG. 1 forreference, the air driver. which is the source of pulsating air, is themost important and unique component of the system. Its construction andperformance will be described later in this disclosure. In general, thesystem functions by providing a constant amount of sinusoidallyalternating electrical power to the air driver unit. The air driver unitis thereby caused to provide a sinusoidally varying flow of air into andout of the patient via the breathing tube and breathing appliance. Thequantity of air actually moved by the air driver is dependent upon thedegree of restriction or impedance provided by the respiratory system ofthe patient. The degree of attenuation of the mechanical motion of theair driver by the patient is then detected, amplified, and analyzed bythe remaining circuitry.

More specifically, again relating to FIG. 1, the oscillator circuitry Igenerates an alternating voltage varying at a rate of cycles per second.This frequency is not critical and signals from 5 cycles per second to50 cycles per second have been utilized in our experiments. This signalis then amplified by a power amplifier 2 to a power level which iscapable of suffi ciently activating the air driver unit. The air driverunit 3 then converts the electrical energy to mechanical motion which isused to drive a small quantity of air into and out of the patient.

The actual extend of mechanical motion occurring within the air driverunit depends upon the loading by the patient. a factor which tends toattenuate and affect the timing of the oscillatory motions within theair driver unit. These motions are detected by a motion sensor devicewithin the air driver and converted to analogous electrical signalswhich describe that motion. These are amplified by a signal amplifier 4and applied simultaneously to two separate detector devices whichanalyze the signal. The first of these is an amplitude detector 6 whichdetermines the overall size of the electrical signal and thereby canprovide information describing the overall impedance of the patient tothe flow of air from the driver unit. The second detector, the phasedetector 5, determines the extent to which the attachment of the patienthas caused the electrical signal detected by the signal amplifier tovary in phase, or be out of step with the electrical energy from thepower amplifier which drives the air driver unit. In order to performthis function, the phase detector must simultaneously be provided withan electrical signal from either the oscillator 1 or power amplifier 2in order to make the timed comparison. In the circuitry we have chosento employ, the amplitude detector 6 also makes use of this timedinformation in order to respond more rapidly to changes in the patientscondition. Phase and amplitude information are then displayed onelectrical meters 7 and 8 and simultaneously the electrical signalswhich describe the phase and amplitude information 9 and 10 areavailable to be applied to any of a variety of recording systems. Themechanical reactive and resistive properties of the patients respiratorysystem at any moment in time can then be calculated from the amplitudeand phase information derived at that moment.

FIG. 2 illustrates one manner in which the data on a given patient canbe presented and examined. Any single point on this graph represents animpedance value. Its direct distance to the origin of the graphrepresents the total or resultant impedance and the angle by which thisline deviates from the horizontal indicates the phase angle. Therefore,by knowing the impedance length and phase angle it is possible to plotthe impedance point. The reactance and resistance values can then bedetermined by projecting this point to the horizontal and vertical axesof the graph.

However, the impedance of the respiratory system in a given patient isnot constant but varies as the patient breathes. FIG. 3 shows a locus ofimpedance points traversed by a patient through a single respirationcycle. The graph indicates that the impedance increases as the patientexhales and decreases as the patient inhales. The dashed lineillustrates the fact that, in this case, the phase angle did notsignificantly change during the respiratory cycle and only the amplitudevaried. This is typical for a normal patient. The resistance to the Howof air into and out of the lungs is primarily generated by the viscosityof the tissues and represents frictional losses of energy. The reactanceis caused by the elastic structures within the chest and lungs and itsvalue is a measure of the amount of energy used to stretch these elasticstructures. FIG. 3 illustrates a normal respiration where the phaseangle remains small and constant during the entire respiratory cycle.

FIG. 4 illustrates an actual case we observed where the phase anglechanged markedly during the respiratory cycle and was relatively high inthe first place. This patient appears to be normal at the inspirationphase of breathing, but the phase angle rises markedly above the normalvalue as the patient exhales. This is the expected curve for a patientwith asthma.

Plotting these curves and producing these graphs by hand is a tediousprocess and it is oftentimes desirable to automatically record anddisplay this information. Commercially available recording systems donot operate in a manner which allows the direct plotting of phase angleand, therefore, cannot produce these graphs using the data from theamplitude and phase detectors.

FIG. 5 illustrates a second embodiment utilizing a revised signalprocessing circuitry where the detectors 5a and 6a determine theresistance and the reactance directly. As before, these detectors alsorequire simultaneous information from the oscillator l or poweramplifier 2. The voltages on the output of these detectorssimultaneously provide resistance and reactance data which are adequateto determine the impedance of the patients respiratory system at anymoment in time.

By applying the voltage from the resistance detector 5a to the circuitrywhich displaces a conventional XY recorder horizontally and the signalsfrom the reactance detector 6a to the circuitry which displaces therecorder vertically, the impedance graph can be plotted automatically asthe patient breathes. In order to generate such a graph, it is onlynecessary to apply the breathing appliance to the patients face,activate the recorder, and allow it to record for the duration of asingle breath. The recorder can then be turned off and the graph iscomplete.

Again referring to FIG. 1, the construction of the air driver unit 3 wehave utilized is illustrated. Externally, this unit appears to be aclosed wooden box with two flexible polyvinyl chloride tubes 11 and 12emerging from one of its faces. A breathing appliance 13 is attached tothe end of one of the flexible tubes 11. Within the box is mounted aspecial audio speaker 14. One Commercially available speaker which issuitable for use is University Sound Model C 8I-IC. The special featuresof this speaker are:

1. It is designed to operate efficiently at very low frequencies. 2. Ithas two identical voice coils which are not electrically connected toeach other, but are mounted on the same speaker cone. 3. These voicecoils are wound so that they are long coils and overlap the magnetstructures on both ends. They thereby provide linear response over wideexcursions of the speaker cone. One of the speaker coils is connected tothe output of the power amplifier 2 and is used to drive the speakercone mechanically. The other coil is used to detect the signal voltagedescribing the actual motion of the speaker cone. This coil is caused tomove in the magnetic field by the motion of the speaker and therebygenerates electrical signals proportional to the velocity of the motion.

The speaker 14 is mounted within the box in such a way that it isinterposed between air chambers 15 and 16. The rear chamber 15 isrelatively large and totally enclosed on five sides by wooden walls andon the sixth side by the rear surface of the speaker 14. In order forthe speaker cone to move it must compress and expand the air in thisrear chamber 15. The front chamber 16 is more complicated and is formedby a series of laminated wooden panels with progressively smaller holescut out of their centers. When assembled, these panels provide a spacewhich gradually becomes smaller in a stepwise fashion as the distancefrom the front of the speaker cone increases. This space funnels down toan exit port into which is inserted a nipple which attaches to thebreathing tube 11 on the outside. There is also a second exit port onthe same surface eccentrically placed into which is inserted anothernipple which is attached to the side tube 12 on the outside of the box.

In order that this system be capable of measuring reactance as well asresistance, it is essential that the system itself exhibit no phaseshift. In order words, the system must be mechanically tuned so that itsmechanical resonance is at the same frequency as the oscillator 1voltage output. Under these conditions, the voltage applied to thedriver unit 3 by the power amplifier 2 will be in phase with the actualmotion of the speaker cone which will, in turn, be in phase with theflow of air out of the breathing appliance 13 which will, in turn, be inphase with the electrical signal detected by the signal amplifier 4.Under these conditions, the reactance of the system, without the patientattached, will be zero and the small impedance that will be measuredwill be caused by the resistance to the flow of air in the tubing. Acomplicating feature in the design is caused by the fact that thepatient must be free to breathe into the system while it is performing ameasurement upon him. This is the reason for the side tube 12 on theequipment.

In theory, it would be ideal if the system would pump air back and forthfreely when the breathing appliance 13 is exposed to the room atmosphereand would undergo no excursion at all when the breathing appliance 13 iscompletely occluded, so that no air can flow through the breathing tube11. Because of the necessity for a side tube 12, the ideal condition 6can only be approximated and requires that the system be tuned for anull as well as for a maximum output condition. The air within the twochambers 15 and 16 is compressible and acts as a reactance which causesthe speaker excursion to lead the applied voltage from the poweramplifier 2 when it is driven in a sinusoidal manner. The air within thebreathing tube 11 and side tube 12 is confined and has finite mass. Whena sinusoidal pulsation is applied, the inertia of the air in these tubesresults in a reactance which causes the motion of the speaker cone 14 tolag behind the driving current from the power amplifier 2. The tuningprocess involves the selection of chamber sizes and tube lengths so thatthese effects exactly balance one another and the net reactance is zero.In order to select the proper dimensions in the design of a specificdriver unit 3, it is advisable to perform the tuning experimentally intwo steps. First, the opening into the breathing appliance 13 is firmlyblocked with a cork or rubber stopper. Under these circumstances, it isdesirable that the speaker 14 move a minimum amount and that thesmallest possible electrical signal be obtained from the sensing coil.Under these conditions, the frequency in which the speaker motion isminimum depends upon the size of the air space in the front chamber 16and the lengths and diameter of the side tube 12. Once as oscillatorfrequency is selected and the design of the box containing the speakerhas been selected, the null can be achieved by selecting the length ofside tube 12 which provides a minimum signal at that frequency. Step 2involves removal of the obstruction from the breathing tube 11 andadjustment of the length of the breathing tube 11 so that a maximumresponse is obtained. The tuning for maximum response depends upon thesize of the rear chamber 15, the length of the breathing tube 11, andthe length of the side tube 12. When the system is properly tuned, thedriving voltage is in phase with the signal voltage when the breathingappliance 13 is occluded and when the breathing appliance 13 is open toroom air. Under these conditions, partial obstruction of the breathingappliance 13 will cause the electrical signal describing the speakermotion to attain a value somewhere between the open and closed positionand will also exhibit no phase shift. Such a partial obstruction isresistive in nature. When the system has been tuned in this manner, itssensitivity to external loading by the respiratory system is maximum andphase shifts can only be produced by external reactances such as thoseexhibited by the respiratory apparatus of the patients under test.

In order to produce maximum transmission of air pulsations through thebreathing tube 11, it is important that there be no large sudden changesin the diameter of the air pathway. It is for this reason that the frontchamber 16 is formed in the shape of a cone by the laminated layers ofwood. FIG. 6 illustrates an external view of the air driver unit 3 justdescribed.

FIG. 7 illustrates an alternative configuration where the side tube isan extension of the breathing tube. While the configuration in FIG. 7 isfeasible and has been shown experimentally to perform in a similarmanner, it is less desirable since the resistance of the tubes plays agreater role and the sensitivity of the system is thereby decreased.

Particular circuits which are suitable for use in accordance with theFIG. 1 embodiment of the invention will now be described.

FIG, 8 shows an oscillator circuit which produces a highly stable l5cycle per second signal with very low-distortion. High-stability andlow-distortion are rigorous requirements of the system because anyslight deviation from the frequency to which the air driver is tunedwill interfere with the detection of the phase difference. FIG. 8 showsan oscillator in which amplifiers 17 and 18 are connected in a feedbackloop.

The tuning is determined by capacitors 19 and 20 and resistors 21, 22and 23. While these components make up a low Q circuit, their inclusionin circuit with the two amplifiers results in a Q multiplication therebyproviding a very stable frequency.

As a unique feature of this circuit, an FET (field effect transistor) 24is interposed in the feedback loop controlling the gain of amplifier l8.Specifically, the F ET 24 is connected to the junction of resistors 25and 26 which are in the feedback path of the amplifier 18. The FET 24presents a variable impedance depending upon the output signal of theamplifier 18. This arrangement causes the amplifier to produce a signalwhich is consistent with the low distortion requirements imposed by thesystem.

The critical circuit parameters are given below:

resistor 23 50K resistor 22 l8K resistor 21 iOK capacitors l9 and 20 lg!resistor 25 27 ohms resistor 26 27K amplifiers l7 and 18 Analog DeviceModel I l l FET 24 2N4856 in an actual embodiment of the invention,power amplifier 2 is a one watt power amplifier of conventionalconfiguration.

Referring to FIG. 9, there is shown the circuitry for the signalamplifier 4 and amplitude detector of FIG. 1.

FIG. 9 shows a signal amplifier 27 of conventional design which providesa variable gain of between and 30 times. The output of the signalamplifier 27 is simultaneously fed to the input of amplifier 28 andthrough diode 31 to the input of amplifier 29. Amplifier 28 is operatedopen loop. The amplified signal from amplifier 27 is of sufficient sizeto cause amplifier 28 to be driven from saturation to cutoff giving asquare wave output signal with an amplitude of 113v. This square waveoutput of amplifier 28 is maintained whether the breathing appliance isopen or completely occluded allowing a constant amplitude signal to beprovided to the input of the phase detector 6.

The output of the signal amplifier 27, in addition to being supplied toamplifier 28 is rectified by diode 31, charges capacitor 33, and is readas a DC voltage by amplifier 29.

As a unique feature of this circuit, the output of amplifier 28 isdifferentiated through capacitor 35 and resistor 36. The differentiatedsignal is then fed to the base of transistor 30 caus ing it to be turnedon once each cycle for the duration of the differentiated output ofamplifier 28. During the time transistor 30 is turned on, capacitor 33is discharged through transistor 30. After transistor 30 returns to itsnonconducting (turned off) state, capacitor 33 charges to a new value.This allows the output of amplifier 29 to maintain a one cycle responsetime to changes in signal amplitude from amplifier 27. Diode 32 is inthe feedback loop of amplifier 29 and changes the gain of amplifier 29to compensate for the forward conducting characteristics of diode 31,therefore, maintaining a linear response at the output of amplifier 29.The output of amplifier 29 is supplied to meter 8. Capacitor 34 isselected for the desired damping of the response of meter 8.

The critical circuit parameters are given below:

amplifiers 27, 28 and 29 Analog Devices Model 1 l l transistor 30 2N3646 diodes 31 and 32 lN270 capacitor 33 2 pf capacitor 34 selected fordesired meter response capacitor 35 0.1 f

resistor 36 4. 7K

Referring to H0. 10, there is shown the circuitry for the phase detector6 of FIG. 1.

Simultaneous signals are fed to FET (field effect transistor) 37 fromthe clipper (amplifier 28 FIG. 9) and from the power amplifier 2 FIG. 1.The square wave signal from the clipper is processed by capacitor 38,resistors 39 and 40, and transistor 41 to achieve a pulse with aduration of l millisecond at the collector of transistor 41. This 1millisecond pulse is fed to the gate connection of F ET 37 causing it toconduct during the pulse duration. The 2v. peak to peak sinusoidallyvarying wave from the power amplifier is fed to the source connection ofthe F ET 37 through a phase shifting network comprised of potentiometer42, resistors 43 and 44, and capacitors 45 and 46. During the time FET37 is conducting, a charge builds up on capacitor 47 resulting in aninput signal to amplifier 48 which is proportional to the phasedifference between the two input signals. Potentiometer 42 is adjustedto give a zero reading on meter 7 when the two input signals are inphase.

Capacitor 49 is selected to give the desired response of meter 7.

The critical circuit parameters are given below:

(field effect transistorlFET 37 2 N 4856 capacitor 38 0.02 ifpotentiometer 39 K resistor 40 20K transistor 4] 2N3646 potentiometer 42100 ohms resistor 43 33K resistor 44 4. 7K

capacitor 45 10 t capacitor 46 50 pi capacitor 47 I pf amplifier 48Analog Devices Model ll l capacitor 49 selected for desired meterresponse The particular combination of the electronic subsystem modules,previously described and illustrated in FIGS. 1, 5, 8, 9 and 10 may haveunique features. However, the most important and unique component of thesystem is the air driver unit 3. The particular design which wasdescribed previously is simply one of many possible designs which allowfor tuning the mechanical system in such a way that it exhibits noreactance until it is applied to a load which has reactive components.It is important that the driver unit itself. have an impedance which isrelatively well-matched to the impedance of the type of tissue beingmeasured, in this case the lungs and chest walls. The technique justdescribed has general applicability to the measurement of the viscousand elastic properties of many body tissues. Similar electronic modulesattached to a mechanical driver unit which is also tuned for zeroreactance can be used to measure the viscoelastic properties of muscles.The purpose of using a mechanical driver unit rather than an air driverunit is to better match the higher impedance of the muscle tissue. Themeasurement of the viscoelastic properties of various muscle groups isimportant in research and in evaluation in the practice ofrehabilitation medicine. It is anticipated that other tissues can alsobe measured using these techniques to provide useful information.

In practice, we have found the phase angle to provide the most importantclinical information. The total impedance, as well as the resistance,can vary markedly during the respiratory cycle, can vary markedly frompatient to patient, and does not seem to be significantly changed undercircumstances where lung disease is present. However, the phase anglewhich reflects the efficiency of the breathing process, is markedlyaffected by disease. Therefore, the significant design feature of thissystem is the fact that it is tuned and the significant performancefeatures are that it can detect reactance and resistance which allow fora complete description of the impedance and an enhanced diagnosticcapability.

We claim:

1. The method of measuring the mechanical reactance and resistance ofbody tissues comprising:

mechanically driving said tissues with a force which varies repetitivelyat a frequency within the range of 5 to 50 cycles per second theresonant frequency of the tissue structure,

detecting the resultant motion of said body tissues, and

comparing said resultant motion with said force to determine theamplitude difference between the two as a measure of the resistiveproperties of said tissues, and

comparing said resultant motion with said force to determine the phasedifference between the two as a measure of the reactive properties ofsaid tissues.

2. The method of measuring the mechanical reactance and resistance ofthe lungs of a patient comprising:

supplying pulsating air, with a force which varies at a frequency withinthe range of to 50 cycles per second, to a breathing appliance coupledto the patients respiratory system,

measuring the movement of air to and from said breathing appliance,

detecting the amplitude of said movement relative to said force at whichsaid pulsating air is supplied to said breathing appliance, saidamplitude being a measure of the resistive properties of the lungs, and

detecting the phase difference of said movement relative to said forceat which said pulsating air is applied to said breathing appliance, saidphase difference being a measurement of the reactive properties of thelungs.

3. Apparatus for the measurement of the resistance and reactance of thelungs comprising:

an air driver unit producing an oscillating source of air in response toenergization thereof,

an oscillator energizing said air driver unit at a frequency within therange of 5 to 50 cycles per second,

a breathing appliance pneumatically coupled to said air driver unit,said air driver unit being mechanically tuned to the frequency of saidoscillator so that the motion of the air in the breathing appliance isin phase with the force applied by said air driver when the breathingappliance is occluded and when the breathing appliance is open, firsttransducer for producing a first electrical signal representing theforce applied to the air in said breathing appliance,

a second transducer producing a second electrical signal representingthe actual motion of the air in said breathing appliance,

measuring circuitry responsive to said first and second electricalsignals for producing an output representing the amplitude of saidmovement relative to said force, and

measuring circuitry responsive to said first and second electricalsignals for producing an output representative of the phase differencebetween said first and second electrical signals, said outputs togetherproviding an impedance profile ofthe lungs.

4. The apparatus recited in claim 3 further comprising:

a side tube pneumatically coupling said air driver and said breathingappliance to the atmosphere so that normal breathing can be accomplishedthrough said breathing appliance.

5. Apparatus for the measurement of the resistance and reactance of thelungs comprising:

an air driver including a diaphragm,

a first coil for driving said diaphragm in an oscillating motion inresponse to an electrical signal applied thereto,

an oscillator for driving said first coil at a frequency within therange of 5 to 50 cycles per second,

a breathing appliance pneumatically coupled to a chamber on one side ofsaid diaphragm,

a second coil mounted on said diaphragm and producing an electricalsignal representing the actual motion of said diaphragm,

a phase detector for comparing the signal applied to said first coilwith the signal appearing on said second coil to determine the relativephase between said two signals, and

an amplitude detector, said signal representing the actual motion ofsaid diaphragm being applied to said amplitude detector to produce anoutput representing the magnitude of the impedance of the inn s. 6. Theapparatus recited in claim further comprising a recorder for displayingthe output of said phase detector relative to the output of saidamplitude detector to display the total impedance profile of the lungs.

7. The apparatus recited in claim 5 wherein said air driver includes anenclosure, said diaphragm being mounted in said enclosure to divide itinto two chambers, said breathing appliance being pneumatically coupledto one of said chambers, said air driver being mechanically tuned to thegiven frequency of said oscillator so that the motion of the diaphragmis in phase with the signal from said oscillator when the breathingappliance is occluded and when the breathing appliance is opened.

8. Apparatus for the measurement of the resistance and reactance of thelungs comprising:

an air driver including a diaphragm,

a first coil for driving said diaphragm in an oscillating motion inresponse to an electrical signal applied thereto,

an oscillator for driving said first coil at a frequency within therange of 5 to 50 cycles per second a breathing appliance pneumaticallycoupled to a chamber on one side of said diaphragm,

a second coil mounted on said diaphragm and producing an electricalsignal representing the actual motion of said diaphragm,

a resistance detector for determining the magnitude of that portion ofthe electrical signal produced by said second coil which is in phasewith the electrical signal applied to said first coil, and

a reactance detector for determining the magnitude and polarity of thatportion of the electrical signal produced by said second coil which isout of phase with the electrical signal applied to said first coil.

9. The apparatus recited in claim 8 further comprising a recorder fordisplaying the output of said resistance detector relative to the outputof said reactance detector to display the total impedance profile of thelungs.

1. The method of measuring the mechanical reactance and resistance ofbody tissues comprising: mechanically driving said tissues with a forcewhich varies repetitively at a frequency within the range of 5 to 50cycles per second the resonant frequency of the tissue structure,detecting the resultant motion of said body tissues, and comparing saidresultant motion with said force to determine the amplitude differencebetween the two as a measure of the resistive properties of saidtissues, and comparing said resultant motion with said force todetermine the phase difference between the two as a measure of thereactive properties of said tissues.
 2. The method of measuring themechanical reactance and resistance of the lungs of a patientcomprising: supplying pulsating air, with a force which varies at afrequency within the range of 5 to 50 cycles per second, to a breathingappliance coupled to the patient''s respiratory system, measuring themovement of air to and from said breathing appliance, detecting theamplitude of said movement relative to said force at which saidpulsating air is supplied to said breathing appliance, said amplitudebeing a measure of the resistive properties of the lungs, and detectingthe phase difference of said movement relative to said force at whichsaid pulsating air is applied to said breathing appliance, said phasedifference being a measurement of the reactive properties of the lungs.3. Apparatus for the measurement of the resistance and reactance of thelungs comprising: an air driver unit producing an oscillating source ofair in response to energization thereof, an oscillator energizing saidair driver unit at a frequency within the range of 5 to 50 cycles persecond, a breathing appliance pneumatically coupled to said air driverunit, said air driver unit being mechanically tuned to the frequency ofsaid oscillator so that the motion of the air in the breathing applianceis in phase with the force applied by said air driver when the breathingappliance is occluded and when the breathing appliance is open, a firsttransducer for producing a first electrical signal representing theforce applied to the air in said breathing appliance, a secondtransducer producing a second electrical signal representing the actualmotion of the air in said breathing appliance, measuring circuitryresponsive to said first and second electrical signals for producing anoutput representing the amplitude of said movement relative to saidforce, and measuring circuitry responsive to saiD first and secondelectrical signals for producing an output representative of the phasedifference between said first and second electrical signals, saidoutputs together providing an impedance profile of the lungs.
 4. Theapparatus recited in claim 3 further comprising: a side tubepneumatically coupling said air driver and said breathing appliance tothe atmosphere so that normal breathing can be accomplished through saidbreathing appliance.
 5. Apparatus for the measurement of the resistanceand reactance of the lungs comprising: an air driver including adiaphragm, a first coil for driving said diaphragm in an oscillatingmotion in response to an electrical signal applied thereto, anoscillator for driving said first coil at a frequency within the rangeof 5 to 50 cycles per second, a breathing appliance pneumaticallycoupled to a chamber on one side of said diaphragm, a second coilmounted on said diaphragm and producing an electrical signalrepresenting the actual motion of said diaphragm, a phase detector forcomparing the signal applied to said first coil with the signalappearing on said second coil to determine the relative phase betweensaid two signals, and an amplitude detector, said signal representingthe actual motion of said diaphragm being applied to said amplitudedetector to produce an output representing the magnitude of theimpedance of the lungs.
 6. The apparatus recited in claim 5 furthercomprising a recorder for displaying the output of said phase detectorrelative to the output of said amplitude detector to display the totalimpedance profile of the lungs.
 7. The apparatus recited in claim 5wherein said air driver includes an enclosure, said diaphragm beingmounted in said enclosure to divide it into two chambers, said breathingappliance being pneumatically coupled to one of said chambers, said airdriver being mechanically tuned to the given frequency of saidoscillator so that the motion of the diaphragm is in phase with thesignal from said oscillator when the breathing appliance is occluded andwhen the breathing appliance is opened.
 8. Apparatus for the measurementof the resistance and reactance of the lungs comprising: an air driverincluding a diaphragm, a first coil for driving said diaphragm in anoscillating motion in response to an electrical signal applied thereto,an oscillator for driving said first coil at a frequency within therange of 5 to 50 cycles per second, a breathing appliance pneumaticallycoupled to a chamber on one side of said diaphragm, a second coilmounted on said diaphragm and producing an electrical signalrepresenting the actual motion of said diaphragm, a resistance detectorfor determining the magnitude of that portion of the electrical signalproduced by said second coil which is in phase with the electricalsignal applied to said first coil, and a reactance detector fordetermining the magnitude and polarity of that portion of the electricalsignal produced by said second coil which is 90* out of phase with theelectrical signal applied to said first coil.
 9. The apparatus recitedin claim 8 further comprising a recorder for displaying the output ofsaid resistance detector relative to the output of said reactancedetector to display the total impedance profile of the lungs.